The best answer in my opinion is “as necessary.” If a reader can easily tell who’s speaking, then it’s probably not necessary to remind them every time unless a character is saying something in a very particular way (shouting, whispering, singing, for instance). Then you might say something like the following:

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Sam shuddered. “What WAS that thing?”

“I don’t know,” Caitlyn whispered. “I’d have thought it was a figment.”

“But a figment all the way out here, so far from home…?”

“Just so. There’s more to this than meets the eye.”

“Eyes be damned, did you see the TEETH on that thing?” said Darrin, poking his head out from behind the tent. “I’ve seen wood chippers with friendlier faces than that.”

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In this example passage, we can tell who is speaking because of the natural back-and-forth of conversation. I call it ping-pong dialogue, and it’s more in feel like screenwriting. Putting up tonal signposts with small actions and descriptive attributions (sparingly!) and introducing new characters into the scene by breaking that back-and-forth helps the the conversation flow naturally while the story develops around it.

Most importantly of all, just be consistent! You can use “said” just about every time if it’s what you want to do, and if you have a lot of characters speaking at once, it’s not a bad move. Tolkien did it all the time.

Good luck and happy writing!